The present invention generally relates to vision correction systems. More particularly, the invention relates to improved methods and systems for determining refractive corrections from wavefront measurements.
Laser eye surgical systems typically employ a system that can track and measure optical characteristics of a patient's eye. One promising eye measurement system uses wavefront technology that allows a surgeon to measure and treat low order and high order aberrations in and on the patient's eye. Wavefront measurement of the eye creates a high order aberration map that permits assessment of aberrations throughout the optical pathway of the eye, e.g., both internal aberrations and aberrations on the corneal surface. The aberration information can then be used to compute a custom ablation pattern for allowing a surgical laser system to correct the complex aberrations in and on the patient's eye. Although such wavefront measurements are often described below in the context of laser surgical systems, such measurements may also be used to formulate refractive correction patterns in alternative eye treatment procedures and systems such as for use in radial keratotomy, intraocular lenses, corneal ring implants, and the like.
One exemplary wavefront technology system is the VISX WaveScan™ System, which uses Hartmann-Shack wavefront sensors that can quantify aberrations throughout the entire optical system of the patient's eye, including first- and second-order sphero-cylindrical errors, coma, and third and fourth-order aberrations related to coma, astigmatism, and spherical aberrations. The aberrations in and on the patient's eye can be displayed to the surgeon in the form of an aberration map.
Wavefront aberrations measured with a wavefront sensor provide a map of optical aberration across the pupil of eye. This aberration map can then be used to plan a refractive correction for improving vision quality via wavefront-guided laser vision correction or other vision correction means. Refractive corrections are often determined by Zernike decomposition of wavefront measurements, as proposed by Liang et al., in Objective Measurement of Wave Aberrations of the Human Eye with the Use of a Harman-Shack Wave-front Sensor, Journal Optical Society of America, July 1994, vol. 11, No. 7, pp. 1-9, the entire contents of which is hereby incorporated by reference.
Typically, one or more parameters of image quality are used to formulate a refractive correction plan from wavefront measurements. Some of the possible image quality parameters include the Strehl Ratio, root mean squared (RMS), the value of individual Zernike terms, fill-width half-height (FWHH) of the point spread function (PSF), and modulation transfer function (MTF). For example, based on a minimized wavefront root mean square (RMS), spherical and cylindrical corrections can be determined by the second-order Zernike terms. This approach is effective when an eye has few high-order aberrations. When an eye has a significant quantity of high-order aberrations, however, minimizing the wavefront RMS does not necessarily lead to best image quality. Additionally, when the eye has a significant amount of spherical aberration, the refractive correction determined by the second-order Zernike terms will vary significantly depending on the pupil size of wavefront data. For an eye with Zernike spherical aberration equal to 0.35 um for a 6 mm pupil, the difference in spherical correction determined by Zernike decomposition can be as large as 0.93 D for pupil diameters varying from 2 mm to 6 mm. This large variation in refractive corrections, depending on the pupil size, contradicts the results seen in clinical refraction and can make the use of wavefront measurements for determining refractive corrections difficult.
Another commonly used parameter is the Strehl Ratio, with a maximized Strehl Ratio often being used to determine refractive correction. The Strehl Ratio, however, covers the total MTF volume up to a cutoff spatial frequency that is typically around 180 cycles/degree for a 6 mm pupil. Spatial frequencies higher than about 60 cycles/degree have little or no bearing on actual vision, because the Nyquist frequency limit dictates that the average retinal receptor is capable of only about 57 cycles/degree. Thus, the Strehl Ratio may include information which is not helpful for refractive correction determination.
Therefore, it would be advantageous to have improved methods and systems for using wavefront measurements of an eye to determine refractive corrections for the eye. Such methods and systems would ideally provide refractive correction patterns which would correlate to vision quality measured in vision tests. Additionally, refractive correction should ideally apply across a variety of pupil sizes. At least some of these objectives will be met by the present invention.